Voice over internet protocol (“VoIP”) communication systems allow the user of a device, such as a personal computer, to make telephone calls across a computer network such as the Internet. These systems are beneficial to the user as they are often of significantly lower cost than traditional telephony networks, such as fixed line or mobile networks. This may particularly be the case for long distance calls. To use a VoIP service, the user must install and execute client software on their device. The client software provides the VoIP connections as well as other functions such as registration and authentication. In addition to voice communication, the client may also provide video calling and instant messaging (“IM”).
One type of VoIP communication system uses a peer-to-peer (“P2P”) network topology built on proprietary protocols. An example of this type of communication system is the Skype™ system. To access the peer-to-peer network, the user must execute P2P client software provided by the operator of the P2P system on their user terminal, and register with the P2P system. When the user registers with the P2P system the client software is provided with a digital certificate from a central server. Once the client software has been provided with the certificate communication can subsequently be set up and routed between users of the P2P system without the further use of a central server. In particular, the users can establish their own communication routes through the P2P system based on exchange of one or more digital certificates (or user identity certificates, “UIC”) to acquire access to the P2P system. The exchange of the digital certificates between users provides proof of the user's identities and that they are suitably authorised and authenticated in the P2P system. Therefore, the presentation of digital certificates provides trust in the identity of the user. It is therefore a characteristic of peer-to-peer communication that the communication is not routed using a central server but directly from end-user to end-user. Further details on such a P2P system are disclosed in WO 2005/009019.
One of the advantages of VoIP communication systems, compared to traditional telephony services provided over the public switched telephone network (“PSTN”), is that presence information can be provided for the users. Presence information is an indication of the current status of a user of the system. More specifically, presence information is displayed in the user interface of the client for each of the contacts that the user has stored, and allows the user to view the current status of the contacts in the system. Example presence states that may be displayed include (but are not limited to) “online”, “offline”, “away”, “not available” and “do not disturb”.
The use of presence states provides a user with prior knowledge of the current state of a contact before attempting to communicate with the contact. For example, if the user is not online, and therefore unable to be contacted, then this is clear to the user before attempting to make a call. Similarly, if a contact is busy and unlikely to answer, then this is also communicated in advance via the presence state. This is a considerable advantage over PSTN telephony systems, which do not provide any prior information on the state of a user. The only option in PSTN telephony is to dial a number and wait and see if it is answered.